Titania-containing extrudate

ABSTRACT

An extrudate comprising titania, a carboxyalkyl cellulose, and a hydroxyalkyl cellulose is disclosed. The extrudates have a smooth outer surface when they exit the extruder. The extrusion processibility is improved.

FIELD OF THE INVENTION

The invention relates to an extrudate comprising titania, a carboxyalkylcellulose, and a hydroxyalkyl cellulose.

BACKGROUND OF THE INVENTION

Titania (or titanium dioxide) is a well-known white inorganic pigment.In addition to being a pigment, titania has other applications. Forexample, titania can be used as a catalyst or catalyst carrier (Stiles,A. B., “Supports Other Than Alumina,” Catalyst Supports and SupportedCatalysts (1987) Butterworths Publishers, pp. 57-85). Commercially,titania is produced as a fine powder. To be used as a catalyst orcatalyst carrier, sometimes it is necessary to form titania intoparticles, such as spheres, tablets, extrudates, and the like. Despitemany efforts in developing methods for producing titania extrudates,many of them are not suitable for commercial production because of theirpoor processibility. Therefore, there is a continued need to develop newprocesses for making titania extrudates, particularly as catalystcarrier (see, e.g., co-pending application [serial number has not yetbeen assigned] filed on Dec. 16, 2009).

SUMMARY OF THE INVENTION

The invention is an extrudate comprising titania, a carboxyalkylcellulose, and a hydroxyalkyl cellulose; and a process for producing theextrudate comprising (a) mixing titania, a carboxyalkyl cellulose, and ahydroxyalkyl cellulose to form a dough; and (b) extruding the dough toproduce the extrudate. The process has improved processibility andproduces extrudates with a smooth outer surface.

DETAILED DESCRIPTION OF THE INVENTION

The invention is an extrudate comprising titania. Suitable titanias canbe rutile, anatase, brookite, or a mixture. Titania may be produced bythe chloride process, the sulfate process, the hydrothermal process, orthe flame hydrolysis of titanium tetrachloride. Examples of suitabletitanias include TiONA® DT-51, DT-52, DT-51D, DT-40, and DT-20 ofMillennium Inorganic Chemicals.

The extrudate comprises a carboxyalkyl cellulose. Cellulose is anorganic compound with the formula (C₆H₁₀O₅)_(n), a polysaccharideconsisting of a linear chain of β-1,4-linkages, as shown in Scheme I,where n=50 to 20,000. Cellulose is the structural component of theprimary cell wall of green plants. Cellulose can be converted into manyderivatives.

A carboxyalkyl cellulose is a cellulose derivative with carboxyalkylgroups bound to some of the hydroxyl groups of the glucopyranosemonomers that make up the cellulose backbone, as shown in Scheme II,where R═H, carboxylalkyl, and m=50 to 20,000. It is often used as itssodium salt, sodium carboxyalkyl cellulose. The functional properties ofcarboxyalkyl celluloses depend on the degree of substitution of thecellulose structure (i.e., how many of the hydroxyl groups have takenpart in the substitution reaction), as well as the chain length of thecellulose backbone and the degree of clustering of the substituents. Theaverage number of substituted hydroxyl groups per glucose unit incellulose derivatives is referred to as the degree of substitution (DS).Complete substitution would provide a DS of 3. Preferably, acarboxymethyl celluloses is used. Preferred carboxymethyl celluloseshave a degree of substitution of 0.5 to 0.9 (D. B. Braun and M. R.Rosen, Rheology Modifiers Handbook Practical Use and Applications (2000)William Andrew Publishing, pp. 109-131). Carboxymethyl celluloses areknown as extrusion aids (U.S. Pat. Nos. 5,884,138 and 6,709,570; U.S.Pat. Appl. Pub. No. 2008/0146721).

The extrudate also comprises a hydroxyalkyl cellulose. A hydroxyalkylcellulose is a derivative of cellulose in which some of the hydroxylgroups in the repeating glucose units are hydroxyalkylated. Some of thehydroxyl groups in a hydroxyalkyl cellulose may also be alkylated. Atypical structure of a hydroxyalkyl cellulose is shown in Scheme II,where R═H, alkyl, hydroxyalkyl, and m=50 to 20,000. Preferably, thehydroxyalkyl group is selected from the group consisting of2-hydroxyethyl, 2-hydroxypropyl, and mixtures thereof. More preferablythe hydroxyalkyl cellulose is alkylated. Most preferably, thehydroxyalkyl cellulose is selected from the group consisting of methyl2-hydroxyethyl cellulose, methyl 2-hydroxypropyl cellulose, and mixturesthereof. Preferably, the degree of methyl substitution is from 1 to 2,more preferably from 1.5 to 1.8; and the 2-hydroxyethyl or2-hydroxypropyl molar substitution is from 0.1 to 0.3. METHOCEL™ K4Mcellulose derivative, a product of The Dow Chemical Company having amethyl substitution of 1.4 and a hydroxypropyl molar substitution of0.21, is preferably used. Hydroxyalkyl celluloses are known as extrusionaids (U.S. Pat. Nos. 5,884,138, 6,316,383, and 6,709,570).

The extrudate may comprise an inorganic oxide other than titania, e.g.,silicas, aluminas, zirconias, magnesias, silica-aluminas,silica-magnesias, zeolites, clays, and the like, and mixtures thereof.Suitable silicas include, e.g., silica gel, precipitated silica, andfumed silica. The weight ratio of other inorganic oxides to the titaniais preferably less than 50:50, more preferably less than 20:80, mostpreferably less than 10:90.

The weight ratio of the carboxyalkyl cellulose to the titania ispreferably from 0.2:100 to 5:100, more preferably from 0.5:100 to 4:100,most preferably from 1:100 to 3:100. The weight ratio of thehydroxyalkyl cellulose to the titania is preferably from 0.1:100 to2.5:100, more preferably from 0.2:100 to 2:100, most preferably from0.5:100 to 1:100. The weight ratio of the carboxyalkyl cellulose to thehydroxyalkyl cellulose is preferably from 5:1 to 1:2, more preferablyfrom 3:1 to 1:1.

To produce the extrudate, the titania, the carboxyalkyl cellulose, andthe hydroxyalkyl cellulose are made into a well-mixed dough. Ifnecessary, a solvent may be used. Suitable solvents include water,alcohols, ethers, esters, amides, aromatic compounds, halogenatedcompounds, and the like, and mixtures thereof. Preferred solvents arewater, alcohols, and their mixtures. Suitable alcohols include methanol,ethanol, isopropanol, tert-butanol, and benzyl alcohol.

A titania sol may be used as the source of the titania. A titania sol isa colloidal suspension of titania particles in a liquid. A titania solcan be prepared by hydrolyzing a titania precursor. Suitable titaniaprecursors include titanium salts, titanium halides, titanium alkoxides,titanium oxyhalides, and the like.

The extrudate of the invention is made by extrusion, a process in whicha dough is pushed through a die or an orifice to create long objects ofa fixed cross-section. Extrusion is commonly used to process plastics orfood, and to form adsorbents, catalysts, or catalyst carriers. Anyconventional extruder may be used. A suitable screw-type extruder isdescribed in “Particle Size Enlargement,” Handbook of Powder Technology,vol. 1 (1980) pp. 112-22.

The carboxyalkyl cellulose and the hydroxyalkyl cellulose are used asextrusion aids. An extrusion aid helps the mixing, mulling, andextruding operation and may improve the mechanical and/or physicalproperties of the extrudate such as crushing strength, surface area,pore size, or pore volume. The extrudate comprising titania, thecarboxyalkyl cellulose and the hydroxyalkyl cellulose has a smooth outersurface. They do not tend to stick to each other while being formed,dried, and calcined, which is suitable for large scale production. Inaddition the combination of the carboxyalkyl cellulose and thehydroxyalkyl cellulose minimizes “feathering.” The term “feathering”means that an extrudate, instead of having a smooth outer surface,exhibits cracks in its surface where small flakes or “feathers” of theextrudate are separated from the surface. “Feathering” not only causesloss of valuable material but also tends to impair the physical strengthof an extrudate.

The extrudate may comprise other extrusion aids, including, e.g., alkylamines, carboxylic acids, alkyl ammonium compounds, amino alcohols,starch, polyacrylates, polymethacrylates, poly(vinyl alcohols,poly(vinylpyrrolidone)s, poly(amino acid)s, polyethers,poly(tetrahydrofuran)s, metal carboxylates, and the like, and mixturesthereof. Preferred poly(alkylene oxide)s are poly(ethylene oxide)s,poly(propylene oxide)s, or copolymers of ethylene oxide and propyleneoxide. Organic extrusion aids are usually removed by calcination.

The extrudate is generally dried after it is formed. The dryingoperation generally removes most of the solvents (e.g., greater than90%) from the extrudate. The drying operation may be performed at 30 to200° C. at atmospheric pressure or under vacuum. The drying may occur inair or an inert atmosphere. Sometimes, it is preferable to raise thedrying temperature slowly so the extrudate will not be cracked orweakened.

The invention includes a calcined extrudate. Preferably, the calcinationis carried out in an oxygen-containing gas to burn off the organicmaterials (e.g., residual solvent and extrusion aids) contained in theextrudate. The calcination may be carried out at 400 to 1000° C., morepreferably from 450 to 800° C., most preferably from 650 to 750° C.Sometimes, it is beneficial to initially calcine the extrudate in aninert gas (e.g., nitrogen, helium) to thermally decompose the organiccompounds contained in the extrudate, and then burn off the organicmaterials in an oxygen-containing gas. Generally, a calcined extrudateafter the calcination contains less than 0.5 wt % carbon. Preferably, itcontains less than 0.1 wt % carbon.

The invention additionally includes a process for producing an extrudatecomprising (a) mixing titania, a carboxyalkyl cellulose, and ahydroxyalkyl cellulose to form a dough; (b) extruding the dough toproduce the extrudate. The process is described in detail in theprevious sections.

Example 1

D-T51 titania (2500 g), a high-purity WALOCEL™ C sodium carboxymethylcellulose (The Dow Chemical Company, 52.5 g), poly(ethylene oxide)(MW=100,000, 35 g), and a 2-hydroxypropyl cellulose (METHOCEL™ K4M, 25g) are mixed in an Eirich mixer for 5 min. Water (1005 g), an aqueousammonium hydroxide (14.8 M, 100 g), and benzyl alcohol (17.5 g) areadded into the mixer. They are mixed for 5 min at the “low” speedsetting, then for 10 min at the “high” speed setting. The dough producedis placed in the hopper of a Bonnot 2-inch extruder (The Bonnot Company)equipped with a die face of 25 holes with a diameter of ⅛ inch. Theextrusion is performed at a rate of approximately 0.25 kg/min. Theextrudates produced have smooth outer surface and there is minimalsticking each other occurring. Almost no feathering is observed.

The extrudates are piled 1 inch deep on a collection tray and dried inair at 80° C. for 12 h, then calcined in air. The calcinationtemperature is raised from room temperature to 500° C. at a rate of 2°C./min, held at 500° C. for 2 h, raised from 500° C. to 700° C. at arate of 10° C./min, held at 700° C. for 3 h, then lowered to roomtemperature.

Some physical properties of the calcined titania extrudate are listed inTable 1. The crush strength of the calcined titania extrudate ismeasured with a Chatillon crush strength analyzer (Model DPP 50). Theforce necessary for failure in 25 measurements is averaged to give thereported value. Bulk density is measured by placing 40 g of the calcinedextrudates in a 100-mL graduated cylinder (1″ nominal outer diameter).The graduated cylinder is tapped until the apparent volume no longerchanges, and then this value is divided into the mass to calculate thebulk density. Voidage is determined by adding the pellets to 50 mL waterin a second graduated cylinder and then tapping until all voids arefilled. The resulting water level is subtracted from the total volume ofthe water and the pellets taken separately to determine the void volumeoccupied by water. Total pore volume is determined by pouring themixture through a sieve basket, shaking to remove excess water and thenweighing the wet extrudates. The increase in mass over the initial 40 gof extrudates divided by the density of water is taken as the measure ofthe pore volume.

Comparative Example 2

The procedure of Example 1 is repeated with the formulation shown inTable 1. The extrudates are droopy as they exit the die face of theextruder and tend to stick to each other as they lay on the collectiontray.

Comparative Example 3

The procedure of Example 1 is repeated with the formulation shown inTable 1. The extrudates are droopy as they exit the die face of theextruder and tend to stick to each other as they lay on the collectiontray.

TABLE 1 Example 1 C. 2 C. 3 Formulation Titania (g) 2500 2500 2500 Water(g) 1005 1250 1005 Poly(ethylene oxide) (g) 35 35 35 WALOCEL ™ Ccellulose (g) 52.5 52.5 52.5 Benzyl alcohol (g) 17.5 17.5 17.5 Ammoniumhydroxide (g) 100 100 100 METHOCEL ™ K4M cellulose (g) 25 0 0 ExtrusionProcessibility good poor poor Properties of Calcined Extrudate Crushingstrength (lb/mm) 1.68 1.37 1.43 Surface area (m²/g) 27.2 n/a n/a Porevolume (mL/g) 0.32 0.30 0.30

Comparative Example 4

The procedure of Example 1 is repeated, except that the formulation isas follows: DT-51 (2000 g), TAMOL™ 1124 dispersant (a hydrophilicpolyelectrolyte copolymer from The Dow Chemical Company, 32.6 g),METHOCEL™ K4M (54.6 g), lactic acid (6 g), water (950 g), aqueousammonium hydroxide (14.8 M, 70 g).

Comparative Example 5

The procedure of Example 4 is repeated, except that alumina (DISPERAL®P2, available from Sasol, 20 g) is used. The extrudates are droopy asthey exit the die face of the extruder and tend to stick to each otheras they lay on a metal tray. The calcined extrudate contains 1 wt %alumina and 99 wt % titania.

TABLE 2 Example Formulation C. 4 C. 5 DT-51 titania (g) 2000 2000METHOCEL ™ K4M cellulose (g) 54.6 54.6 TAMOL ™ 1124 dispersant (g) 32.632.6 Water (g) 950 950 Ammonium hydroxide solution (g) 70 70 DISPERAL ®P2 alumina (g) 0 20 Lactic acid (g) 60 60 Extrusion Processibility poorpoor

Example 6

The procedure of Example 1 is repeated, except that the formulation isas follows: DT51 (300 g), TAMOL™ 1124 dispersant (5 g), WALOCEL™ Ccellulose (6 g), METHOCEL™ K4M cellulose (6 g), lactic acid (4.5 g),water (155 g), and aqueous ammonium hydroxide (14.8 M, 11 g). Theextrudates have smooth outer surface. Minimal feathering is observed.Almost no extrudate is observed to stick to others.

Comparative Example 7

The procedure of Example 6 is repeated, except that the formulation isshown in Table 3. The extrudates slump upon exiting the die. They stickto each other on the collection tray.

Comparative Example 8

The procedure of Example 6 is repeated, except that the formulation isshown in Table 3. The extrudates do not tend to stick to each otherafter laying the on the collection tray. However, they appear to befeathering.

TABLE 3 Example Formulation 6 C. 7 C. 8 DT-51 titania (g) 300 300 300Water (g) 155 145 145 TAMOL ™ 1124 dispersant (g) 5 5 5 WALOCEL ™ Ccellulose (g) 6 0 6 Lactic acid (g) 4.5 4.5 4.5 Ammonium hydroxide (g)11 11 11 METHOCEL ™ K4M cellulose (g) 6 6 0 Extrusion Processibilitygood poor poor

Example 9

NaHCO₃ powder (27 g) is slowly added to an aqueous solution containingNa₂PdCl₄.3H₂O (31.4 g), NaAuCl₄.2H₂O (11.3 g), and water (235.4 g). Themixture is stirred at room temperature for 10 min. The solution issprayed with a pipette on calcined titania extrudates prepared inExample 1 (1000 g) while they are being tumbled in a rotating flask.Once the impregnation is finished, the rotating flask is heated to about100° C. with a heat gun. The impregnated extrudates are tumbled foranother 30 min at 100° C., then placed in an oven at 80° C. for 2 hbefore they are cooled to room temperature.

The dried extrudates are washed with warm water (50-80° C.) until nochloride can be detected by mixing the wash filtrate solution with a 1wt % silver nitrate solution to observe precipitation. After washing isfinished, the catalyst is dried at 80 to 100° C. to remove water. Thenthey are heated at 230° C. for 3 h in air, and at 230° C. for 30 minunder a nitrogen flow. The temperature is raised to 500° C. under a flowof 10 mol % hydrogen in nitrogen gas, and held for 3 h before it iscooled to room temperature.

The extrudates are washed with an aqueous solution containing 10 wt %potassium acetate and 1 wt % potassium hydroxide (10 L). The washedextrudates are dried under nitrogen at 125° C. for 2 h. A palladium-goldcatalyst is obtained. It contains 0.93 wt % Pd, 0.54 wt % Au, and 1.5 wt% K.

Example 10

The palladium-gold catalyst prepared in Example 9 is tested for vinylacetate production in a fixed-bed reactor (stainless steel, 1 inchO.D.). The reactor is charged with a mixture of the catalyst (10 g) andan inert alpha alumina cylindrical pellets (⅛″ in diameter, surface area4 m²/g, pore volume 0.25 mL/g, 25 g). The feed contains 46.1 mol %helium, 33.9 mol % ethylene, 11.48 mol % acetic acid, 4.2 mol % oxygen,and 4.2 mol % nitrogen. The reactor pressure is 80 psig and the spacevelocity relative to the volume of the catalyst is 3050 h⁻¹ at standardtemperature and pressure. The reactor is cooled using a fluidized sandbath, the temperature of which is set at 130° C. The product stream isanalyzed by gas chromatography (GC). Oxygen conversion, oxygenselectivity, oxygen yield to vinyl acetate, and ethylene selectivity tovinyl acetate between 75 to 100 h on stream are calculated from the GCresults and listed in Table 4. Oxygen conversion is calculated bydividing the amount of oxygen consumed by the total amount of oxygen fedto the reactor. Oxygen selectivity to vinyl acetate is the amount ofoxygen consumed in making vinyl acetate divided by the total amount ofoxygen consumed. Oxygen yield to vinyl acetate is the product of oxygenconversion multiplied by oxygen selectivity. Ethylene selectivity tovinyl acetate is the amount of ethylene consumed in making vinyl acetatedivided by the total amount of ethylene consumed. Catalyst productivityis the grams of vinyl acetate produced per liter of the catalyst perhour.

TABLE 4 Oxygen conversion (%) 63.7 Oxygen selectivity to vinyl acetate(%) 86.3 Oxygen yield to vinyl acetate (%) 54.9 Ethylene selectivity tovinyl acetate (%) 97.4 Catalyst productivity (g · L⁻¹ · h⁻¹) 492.2

1. An extrudate comprising titania, a carboxyalkyl cellulose, and ahydroxyalkyl cellulose, wherein the weight ratio of carboxyalkylcellulose to titania is from 1:100 to 3:100 and the weight ratio ofhydroxyalkyl cellulose to titania is from 0.5:100 to 1:100.
 2. Theextrudate of claim 1 wherein the carboxyalkyl cellulose is acarboxymethyl cellulose.
 3. The extrudate of claim 1 having acarboxyalkyl cellulose to hydroxyalkyl cellulose weight ratio of from5:1 to 1:2.
 4. The extrudate of claim 1 having a carboxyalkyl celluloseto hydroxyalkyl cellulose weight ratio of from 3:1 to 1:1.
 5. Theextrudate of claim 1 wherein the hydroxyalkyl cellulose is selected fromthe group consisting of 2-hydroxypropyl cellulose, 2-hydroxethylcellulose, and mixtures thereof.
 6. The extrudate of claim 1 wherein thehydroxyalkyl cellulose is selected from the group consisting of methyl2-hydroxypropyl cellulose, methyl 2-hydroxethyl cellulose, and mixturesthereof.
 7. The extrudate of claim 1 wherein the titania constitutes atleast 10 weight percent of the extrudate.
 8. The extrudate of claim 1further comprising an inorganic oxide other than titania.
 9. Theextrudate of claim 8 having an inorganic oxide to titania weight ratioof less than 10:90.
 10. The extrudate of claim 1 having a smoother outersurface than an extrudate not comprising the carboxyalkyl cellulose andthe hydroxyalkyl cellulose.
 11. The extrudate of claim 1 having lessfeathering than an extrudate not comprising the carboxyalkyl celluloseand the hydroxyalkyl cellulose.
 12. The extrudate of claim 1 wherein theextrudate sticks to each other less than an extrudate not comprising thecarboxyalkyl cellulose and the hydroxyalkyl cellulose.
 13. The extrudateof claim 1 is more suitable for large scale production than an extrudatenot comprising the carboxyalkyl cellulose and the hydroxyalkylcellulose.